Substantial evidence suggests that oxidative stress and mitochondrial dysfunction may play a role in neurodegeneration associated with HD. However, the precise mechanism(s) by which mutant huntingtin (htt) causes mitochondrial dysfunction remain largely unknown. We recently demonstrated a regulatory role of Ape1, the major mammalian apurinic/apyrimidinic (AP) endonuclease that participates in the base excision repair (BER) pathway, on mitochondrial function. Thus, our main objective is to determine the mechanisms of Ape1-mediated mitochondrial dysfunction in the context of mutant htt. This proposal will test the hypothesis that mutant htt, in combination with age-related effects, mediates mitochondrial dysfunction and neurodegeneration by targeting Ape1, which in turn results in deficient repair of mtDNA. We propose to test our hypothesis using a combination of in vivo and in vitro models of HD and directly test our hypothesis by determining if: 1) age-related changes in Ape1 synaptic nerve terminals contribute to mtDNA damage, mitochondrial dysfunction and neurodegeneration in HD and 2) what mechanism(s) might trigger Ape1-associated mitochondrial dysfunction in the context of the htt mutation. This study is likely to provide insight into a possible regulatory mechanism of Ape1 in mutant htt-induced mt dysfunction and neurodegeneration.